In the technology of flat panel displays the trend in reduction of size is resulting in interconnections that must be made to thin film pads on the display that have low contact resistance and that require a much smaller interconnection area than has been required for the present technology.
Heretofore in the art the technology of flat panel displays employs the technique of surface mounting technology wherein a thermal bonding flex tape forms the interconnecting member between a printed circuit card and the pads on the display. While the attachment of a conductor to the printed circuit card uses a conventional solder reflow method of lead bonding any attachment of a conductor to the ever more finely spaced pads on the substrate of the display requires a much more sensitive bonding technique.
The current state of the art uses an anisotropic conductive polymer as the bonding media instead of the solder material. The pads on the substrate require materials that have oxide films that are difficult to remove by the chemical fluxes used in connection with solder materials. Additionally, the soldering operation requires high temperature excursions and dwell at those temperatures which exposes the delicate structures and materials in a flat panel display assembly to severe damage.
A key technical limitation at this point in the advancement of the art is the presence of higher than tolerable contact resistance which is inherent in the current bonding technology.
A potential solution to the high contact resistance problem would be to employ the technique of diffusion bonding where there is diffusion of atoms from the two surfaces of the materials being bonded across the bonding interface. Ideally the lowest contact resistance would be provided by diffusion bonds under conditions where any thin oxides on the pads are penetrated and intimate contact between the two mating surfaces provides atomic bonding which has very low contact resistance and very high strength. The ability to achieve good diffusion bonds however is affected by some aspects of the pad materials metallurgy that must be used in many flat panel display constructions. The metallurgy is usually aluminum, or molybdenum or a layer structure of Mo/Al/Mo, and it is relatively thin, usually of the order of 1 micrometer. With this metallurgy, for diffusion bonding to take place the tenacious thin oxide films known to form on aluminum or molybdenum surfaces have to be penetrated and the proper level of thermal energy must be present for diffusion to occur at the interface.
The application of the necessary level of thermal energy also must be achieved within certain considerations. Since heat and pressure alone will not penetrate the oxide films, the use of a single point tip heated electrically, or hot thermode elements that press on a row of connections simultaneously, will not be sufficient to form bonds. Ultrasonics have been found to be ineffective because the high levels of vibratory motion needed to provide the energy to clean the bonding surfaces sufficiently to promote diffusion causes structural damage. Further, in displays in which there is a glass substrate, the material is brittle and subject to cracking under bonding stresses and thermal or mechanical shock.
The use of concentrated short duration combined thermal and vibration energy such as would be provided by combining a laser with an ultrasonic source in a single tool would appear to be a promising technology for the control needed to perform the bonding under the size, vibration and thermal sensitivity limitations.
The use of the combination of laser and ultrasonic energy has been developing in the art. U.S. Pat. Nos. 4,330,699 and 4,534,811 are examples. As the requirements of the art have become more stringent, a tool employing the use of a shutter between a continuous wave laser and an optical fiber that was terminated in a tip designed to hold a wire in wire bonding, was developed by Chalco et al. The development is described in an article entitled "Discrete Wire Bonding Using Laser Energy" in Semiconductor International, May 1988, Pages 130-131. Further developments in the art involved tighter limitations on bonding pressure and temperature control where the substrate can move and there are substrate melting limitations. These limitations are discussed in U.S. Pat. No. 4,970,365 wherein in a tool employing a synchronized ultrasonic pulsed continuous wave laser with an optical fiber delivers shaped focused energy through a center hole in a slotted bonding tip.
Further control of any energy in excess of the minimum necessary to overcome the oxides and produce diffusion at the contact interface is needed in order to meet all the limitations being imposed by the use of brittle substrates and ever more complex metallurgy.